1. Field of the Invention
This invention relates to a method of heat treatment of a Groups II-VI compound semiconductor, in particular, the method of heat treatment for purifying the Groups II-VI compound semiconductor and the heat treatment method for producing a Groups II-VI compound semiconductor of a high degree of purity single crystal.
2. Discussion of Background
Light-emitting semiconductor elements (for example, Light-Emitting Diodes (LEDs)) which have light-emitting zones from red to green using compound semiconductor materials are in mass-production, and further developments are in progress in order to improve their brightness. However, no satisfactory production techniques have yet been provided for blue color light-emitting elements which have blue light emitting zones in the visible region.
For the production of a blue light-emitting element, it is necessary that the forbidden band (bandgap) of the semiconductor material should exceed 2.6 eV. Recently attention has been drawn to Groups II-VI compound semiconductors as semiconductors which satisfy this requirement. Amongst these, zinc sulphide (ZnS), zinc selenide (ZnSe) and mixed crystal (ZnSSe) whose forbidden bands are with the 2.6-3.7 eV region are viewed as promising. Hereafter, the above material is referred to by the general term of zinc sulphide selenide (ZnS.sub.x Se.sub.1-x ; 0.ltoreq.x.ltoreq.1). For the methods of growing the above-mentioned single crystal, there are the high-pressure melting methods which are known as crystal growth methods under conditions of high temperature and high pressure, for instance, the Bridgeman method, the Tammann method, the sealed tube chemical transport method, and the sublimation method. Using the above methods, it is possible to obtain single crystal with a comparatively large diameter.
However, the materials used for growing the above single crystal are mostly in powdered form or are irregular polycrystal materials. That is to say, the inclusion factors by wt ppm in an example of such a material have been measured as Al, Si:0.5, Cu, Cr:0.3, Mg:0.1. Since, for this reason, single crystal made from this kind of raw material naturally inherits impurities, crystal which would satisfy the purpose of use could not be obtained. Also, since these impurities have a complex relationship with lattice defects, there were problems such as limitations in the production of light-emitting elements from this crystal. Furthermore, there are no detailed reports of prior extraction of impurities for these materials.
As described in the above-mentioned prior art, since large amounts of impurities such as copper, aluminum and magnesium were included in the compound semiconductor crystal used for light-emitting elements, the single crystal made from these raw materials had poor purity, for instance, in single crystals such as ZnS and ZnSe, there were problems of controlling the electrical resistance and the type of conductivity, and the problem of obstruction of the light-emitting characteristic. For the above reasons there have been great hindrances to the achievement of blue light-emitting elements with superior performance.